1. Field of the Invention
This invention relates to the production of synthetic fuels and is particularly concerned with a process wherein a reactant gas comprising carbon oxides and hydrogen is methanated to produce a substitute pipeline gas having a heating value of about 900 to 1000 BTU per standard cubic foot.
2. Description of the Prior Art
In the fixed bed catalytic methanation of gases containing carbon monoxide and hydrogen, the reaction between the carbon monoxide and hydrogen initiates generally at 400.degree. F. to 450.degree. F. The reaction is very exothermic and, if not controlled within the reactor, can cause sintering of the catalyst, carbon deposition on the catalyst and/or thermal cracking of the product methane to carbon and hydrogen. Carbon formation through thermal cracking and/or carbon monoxide disproportionation in turn has a tendency to foul the catalyst bed.
Most prior art methanation catalysts, by the nature of their supports, are not hydrothermally stable when used on a continuous basis at temperatures in excess of 900.degree. F. to 1000.degree. F. Therefore, many adiabatic prior art processes are "lower temperature" processes and preserve the catalyst by limiting methanation to temperatures from about 550.degree. F. to about 900.degree. F. For example, most of these "lower temperature" methanation processes employ nickel catalysts which tend to deactivate at 900.degree. F.
Also, it is important that the gas enter at the lowest inlet temperature which will give an acceptable initiation reaction rate and still prevent the formation of a carbonyl compound which can occur through the reaction of the carbon monoxide with the catalyst at temperatures below proper operating temperatures.
To overcome some of these problems caused by overheating or carbonyl formation in these "lower temperature" methanation processes, extensive recycle streams are used as a diluent to absorb some of the exothermic heat evolved. Additional measures for avoiding too high temperatures in the reactor include cooling of the catalyst bed or of the reaction gases. For example, direct cold gas recycle and internal cooling of the reactor by heat transfer surfaces within the bed are recognized methods by which temperature controls may be effected. Local heating is difficult to avoid when using the latter and the building of internal exchange surfaces tends to be expensive. The hot gas recycle and direct cold gas recycle methods, on the other hand, require high recycle ratios. As a consequence, large pressure drops through the catalyst beds occur and the requirements for compressor power and stricter design specifications increase proportionately, hence increasing compression construction costs.
There is also a "higher temperature" prior art adiabatic methanation process which limits methanation temperatures between 900.degree. F. and 1600.degree. F. to preserve activity of the steam-hydrocarbon reforming catalyst employed therein. Below about 900.degree. F., this "higher temperature" catalyst is essentially non-functional for methanation. Consequently, in order to initiate the exothermic methanation and water-gas shift reactions, the temperature of the steam-reactant gas feed stream fed to the first primary wet reactor must be above about 900.degree. F. This "higher temperature" methanation process eliminates the disadvantages associated with "lower temperature" processes, i.e., the need to recycle product gases or introduce other gas streams for the purpose of diluting the reactants to control temperature, and in addition, it generates high pressure steam which may be used to serve other process needs.
However, preheating the steam-reactant gas feed stream fed to the first primary wet reactor to a temperature above about 900.degree. F. requires the expenditure of large amounts of heat energy and/or the use of equipment having extensive heat transfer surfaces and materials of construction which will withstand the high temperatures and pressures employed to superheat steam and/or the feed stream. Moreover, there is the disadvantage that during preheating at temperatures above about 500.degree. F., carbon may be deposited on heat exchange surfaces.